Solar collectors are devices designed to convert solar radiation into heat that can be used to perform work.
One design of one type of solar collector known as the flat plate solar collector is illustrated schematically in FIG. 1 of the drawings. Referring to FIG. 1, the flat plate solar collector includes a housing 100 comprising a transparent cover plate glass 102. In use, solar radiation enters the housing 100 through the cover plate glass 102 and strikes an absorption plate 110 located within the housing 100. The absorption plate 110 may be coated with a material capable of absorbing solar radiation and converting the solar radiation into heat. The flat plate collector includes a pipe array 106 which is bonded to the absorber plate 110 such that a heat transfer fluid entering the array 106 at entry point 104, is subsequently heated, and emerges at exit point 108 at a higher temperature. The space between cover plate glass 102 and the absorber plate 110 is usually filled with air. The space below the absorber plate 110 is usually filled with an insulating material 112.
The performance of the flat plate collector, in terms of maximum achievable output temperature of the heat transfer fluid, is limited to a large extent by thermal losses. These losses can occur via radiation from the pipe array 106 and from the absorber plate 110. The thermal losses can also occur via convection through the air disposed between the absorption plate 110 and the cover plate glass 102. Finally, the thermal losses can occur via conduction through the insulating material 112. The dominant losses are via convection and conduction. Typical maximum operational temperatures reached by the heat transfer fluid after thermal losses are about 120° C.
Another design for a solar collector is known as an evacuated tube array. This design is illustrated schematically in FIG. 2 of the drawings. Referring to FIG. 2, the evacuated tube array comprises a collection of evacuated tubes 202, one of which is shown in cross-section. As will be seen from the cross-section, each tube 202 comprises an outer transparent shell 210, which surrounds an inner absorbing shell 212. In use, solar radiation passes through the outer shell 210 and impinges on an inner absorbing shell 212 where it is absorbed and converted to heat. The inner shell 212 and the outer shell 210 are separated by a vacuum and have no internal supports with the exception of a mechanical support at one end of the tube. Heat is transferred via a heat transfer fluid circulating through an internal pipe 214. Each of the internal pipes of the individual tubes 202 within the evacuated tube array is connected such that heat transfer fluid entering at entry point 204, collects heat from all of the tubes 202 and emerges at a higher temperature at exit point 206.
The performance of evacuated tubes 202 is also limited by thermal losses. In this case, however, the losses are not dominated by convection or conduction because of the presence of a vacuum, and the small number of heat conducting internal supports. Instead, thermal losses in the evacuated tubes 202 evacuated tube array design are dominated by radiation losses from the evacuated tubes 202. These losses increase as the temperature of the evacuated tubes 202 increases according to classical blackbody theory. Typical maximum operational temperatures of the heat transfer fluid for the evacuated tube array design are about 200° C.